The present invention relates to a voltage controlled oscillator usable in the field of a wireless installation such as a portable telephone and an automobile telephone and also usable in the field of various electric installations. More particularly, the present invention relates to a voltage controlled oscillator suitable for a communication system in which a plurality of frequency bands are used.
Concerning the conventional voltage controlled oscillator having a function of changing over the oscillating frequency band, there are provided a first conventional example shown in FIG. 5 and a second conventional example shown in FIG. 6. The first conventional example is described in Japanese Postexamined Utility Model Publication 1-27302 as a conventional example. Also, the first conventional example is described in Japanese Unexamined Utility Model Publication 61-64723 as a conventional example. In this first conventional example, a PIN diode is used. When a voltage impressed upon this PIN diode is turned on and off so that a constant of a peripheral element of the oscillating transistor can be changed, the oscillating frequency band is changed over.
The first conventional example shown in FIG. 5 will be explained below in more detail. In the drawing, C0 to C9, Ca and Cb are capacitors, R1 to R7 are resistors, L0 and L1 are coils, S1 is a strip line composing a resonator, Cv is a variable capacitance diode, Q1 and Q2 are transistors, and Pd is a PIN diode. Reference numeral 1 is a power supply terminal, reference numeral 2 is a frequency control terminal, reference numeral 3 is an output terminal for an oscillated signal, and reference numeral 4 is a changeover terminal of the oscillating frequency band. Transistor Q1 and its peripheral section compose an oscillating stage 5, and transistor Q2 and its peripheral section compose a buffer stage 6.
Operation of the voltage controlled oscillator shown in FIG. 5 will be explained as follows. The oscillating stage 5 is set into oscillation as follows. Electric power is supplied from the power supply terminal 1 to transistor Q1. Further, an arbitrary intensity of voltage is supplied from the frequency control terminal 2 to variable capacitance diode Cv via coil L0. At this time, the oscillating stage 5 is set into oscillation, wherein the oscillating frequency is determined by a relation between the capacitance of the variable capacitance diode Cv and the constants of other capacitors of the oscillating stage 5 and the strip line S1.
At this time, when a voltage is applied upon the oscillating frequency changeover terminal 4, the PIN diode Pd is turned on. Therefore, capacitor Cb is electrically neglected, and the grounding capacitance provided in parallel with strip line S1 is composed of only capacitor Ca.
On the other hand, when a voltage is not applied upon the oscillating frequency changeover terminal 4, the PIN diode Pd is in off-state. Therefore, the grounding capacitance provided in parallel with strip line S1 is composed of a composite capacitance of series connection in which capacitors Ca and Cb are connected in series. This composite capacitance (Ca.multidot.Cb/(Ca+Cb)) is necessarily smaller than the capacitance of capacitor Ca.
Therefore, it is possible for the oscillating stage 5 to oscillate two oscillating bands in accordance with the voltage applied upon the oscillating frequency band changeover terminal 4. In this case, the oscillating frequency band in the case of applying a voltage upon the oscillating frequency band changeover terminal 4 is lower than the oscillating frequency band in the case of not applying a voltage upon the oscillating frequency band changeover terminal 4.
A signal generated in the oscillating stage 5 is inputted into the buffer stage 6 via capacitor C5. In the buffer stage 6, electric power is supplied from the power supply terminal 1 to transistor Q2, so that transistor Q2 can be driven. The buffer stage 6 functions as follows. The buffer stage 6 amplifies an inputted signal and stabilizes the oscillating operation in the case of mismatching of impedance between the oscillator and the outside of the oscillator. Also, the buffer stage 6 stabilizes the oscillating operation in the case of fluctuation of impedance.
As shown in FIG. 6, there is proposed a system in which two sets of voltage controlled oscillator, the oscillating frequency bands of which are different from each other, are used, and they are changed over so as to output a signal.
Operation is conducted as follows in the second example shown in FIG. 6. Reference numerals 7 and 8 are voltage controlled oscillators which respectively oscillate signals of different frequencies. When one voltage controlled oscillator 7 is used, it is connected with the power supply terminal 1, frequency control terminal 2 and output terminal 3 via the respective switches 9, 10 11. When the other voltage controlled oscillator 8 is used, the switches 9, 10, 11 are changed over to the side of the voltage controlled oscillator 8.
In the conventional example shown in FIG. 5, the following problems may be encountered. It is possible to oscillate signals of different frequency bands when constants of some elements are changed by switching operation. However, according to the above arrangement, it is difficult to oscillate a signal by changing over the frequency band between two frequency bands in such a manner that one frequency band is twice as high as the other frequency band.
In the first conventional example, in the design of C/N characteristic, which expresses an intensity of phase noise of an oscillating signal, which is an important characteristic of the voltage controlled oscillator, if C/N characteristic is optimized in one of the frequency bands, it becomes difficult to optimize C/N characteristic in other frequency band. That is, it is difficult to design C/N characteristic to be optimized in both frequency bands. Therefore, the design of C/N characteristic is necessarily conducted to take trade-off optimizing conditions in both frequency bands. For the above reasons, it is impossible to put the above frequency controlled oscillator into practical use in which a high C/N characteristic is required.
When the voltage controlled oscillators are mass-produced, the oscillating frequencies of the voltage controlled oscillator are uneven in a wide range. Therefore, it is necessary to adjust the oscillating frequencies. When the oscillating frequency is adjusted so as to ensure a necessarily high accuracy of the oscillating frequency, which is not higher than 0.1% with respect to the oscillating frequency of a usual mass-produced product, it is necessary to obtain the accuracy by both frequencies to be changed over. However, since the respective oscillating frequencies are oscillated by the same oscillating stage 5, when one oscillating frequency is adjusted, it is necessary to watch the other oscillating frequency. For the above reasons, the method of adjusting the oscillating frequency is complicated.
On the other hand, in the second conventional example, there are respectively provided independent voltage controlled oscillators. Therefore it is possible to solve the above problems caused when the same oscillating stage is used. However, in the second conventional example, two sets of voltage controlled oscillators are used. Accordingly, there is a tendency that the size of the functional block of the second conventional example becomes larger than that of the first conventional example described before.
Further, in the second conventional example, it is necessary to provide a plurality of switches for changing over the terminals of the voltage controlled oscillators. In order to change over the switches, it is necessary to provide a circuit. Therefore, the size of the functional block of the second conventional example is further increased.